Answer
$7.03\lt\sigma\lt12.52$
Work Step by Step
Given $n=25,s=9$, at a 95% confidence and $df=24$ the critical values are $\chi^2_{left}=12.40 $ and $\chi_{right}^2=39.36 $
and the interval of the population standard deviation can be found as
$\sqrt {\frac{(n-1)s^2}{\chi_{right}^2}}\lt\sigma\lt\sqrt {\frac{(n-1)s^2}{\chi_{left}^2}}$ which gives $7.03\lt\sigma\lt12.52$
